"The evidence that Google+ wasn't ready to be a Facebook killer is that it is invitation only."
Google knows the best way to release a product - especially one in unfamiliar territory like the social space - is not to flip a giant switch and hope that when the whole world starts using it, nothing goes wrong. For one, the scalability issues when you're Google are enormous: they know they'll have tens of millions of 7-day users the first 7 days. You don't want to screw that up. That doesn't even touch on product design.
But taking a random sample of people and getting a hopefully-large-enough subset of their friends is not what you want for a social product: you want all of your friends in it, or you just get frustrated and go back to your previous solutions. Facebook /nailed this/ with their multi-year long staged rollout by targeting colleges: already self-contained social ecosystems where almost all of a student's friends are going to be other students; you start with one college, then two, then slowly ramp up until you can flip the big switch. Google simply does not understand this, and keeps falling back to their experience with Gmail, which was a fundamentally different beast.
"Everyone in the City of New York now has Google+: enjoy!"
(Also, I'm somewhat confused by the question, as I specifically demonstrated something that is akin to a suggestion by way of demonstrating what Facebook did right... why are you asking this? There are a million ways to do "invite closed communities rather than random people", like allowing Google Apps domains to join rather than people: that way you at least could get "my company/organization circle" or something; you could imagine inviting e-mail domains, so "anyone with ucsb.edu e-mail addresses now gets on", etc.: the problem is that I now know a small handful of my friends who are using Google+, but as most of my friends can't I will rapidly tire of posting there when I can post to Facebook.)
Google knows the best way to release a product - especially one in unfamiliar territory like the social space - is not to flip a giant switch and hope that when the whole world starts using it, nothing goes wrong. For one, the scalability issues when you're Google are enormous: they know they'll have tens of millions of 7-day users the first 7 days. You don't want to screw that up. That doesn't even touch on product design.